The present invention relates to a method for compensating for a mechanical oscillation, in particular for a rotational oscillation, which has a frequency spectrum which can be approximately represented by a number of discrete frequency components, at a machine shaft, in particular in a printing unit or in a printing machine, via at least one actuator acting directly or indirectly upon the machine shaft.
Moreover, the present invention relates to a printing unit having at least one machine shaft driven by an actuator and a measuring device for picking up the mechanical oscillation of the machine shaft.
A mechanical oscillation is to be understood both as periodic changes of one or more coordinates of the machine shaft, for example, rotational oscillations, vibrational oscillations or the like, and superpositions of a plurality of periodical changes. The term machine shaft chosen hereinafter denotes a mathematical axis of rotation and, in this context, can be, in particular, any shaft, axle, machine axis, roll, cylinder, or the like, of a machine. In this connection, the term machine shaft will include hereinafter also a virtual axis of rotation, that is a signal calculated from coordinate values of one or a plurality of axes of rotation, in particular the difference between coordinates of two real machine shafts.
In the case of machines having speed or positional control of at least one machine shaft, it is possible that changing torques act upon the controlled shaft because of cam discs, imbalances, or other constructional conditions. Depending on the transmission ratio between the shaft of the original torque and the controlled shaft, oscillations are superposed on the latter whose frequency is proportional to its rotational speed, i.e., has a fixed machine order. Moreover, excitations having a fixed frequency which is independent of the speed of the controlled shaft can be superposed upon the rotary motion. In both situations, the frequency spectrum of the mechanical oscillation of the position, speed and acceleration of the controlled shaft contains disturbing discrete frequencies which will be referred to as disturbances hereinafter, independently of their origin. If the intention is for the machine shaft to rotate at a constant speed, then disturbances give rise to deviations from the setpoint speed or from the rotational setpoint angle which cannot be completely corrected but only reduced to a sufficient degree with increased outlay in connection with the driving mechanism.
Especially in the case of printing machines, whether they are sheet-fed printing presses or web-fed printing presses, the accuracy with which the speed of the controlled shaft is maintained or with which a rotational setpoint position is followed, has a decisive influence on the quality of the product. Disturbances having an integral frequency ratio to the rotational frequency of the controlled shaft of a paper guiding cylinder are generally negligible since they are equal for all print images or print sheets. However, all other frequencies are disturbing because they can give rise to the so-called xe2x80x9cghostingxe2x80x9d when, in other words, successive sheets are printed at an offset relative to one another.
If a sheet-fed printing press is composed of a plurality of separately driven, mechanically decoupled parts such as printing units, printing unit groups, or the like, then variations in the angular difference between two successive paper guiding cylinders, that is two controlled shafts, at the sheet transfer between the parts of the printing machine become directly noticeable as variations in the circumferential register. In this context, it is desired for the transfer angle to be identical from sheet to sheet; however, the transfer angle is impaired by oscillations of non-integral order.
Diverse devices and methods for damping mechanical oscillations, in particular rotational oscillations, in printing machines are already known.
European Patent EP 0 592 850 B1 describes a device and a method for damping mechanical oscillations of printing machines. This device features at least one actuating member and one vibration sensor; these can be arranged in a control loop as well. In this context, the damping of printing quality impairing mechanical oscillations in the stock-guiding system of a printing machine is directed to the compensation for asynchronous oscillations, that is only for oscillations which occur non-periodically with the revolutions of the rotating parts.
German Patent Application DE 44 12 945 A1 discloses a device and a method for damping mechanical oscillations of printing machines which also allows integral oscillation orders to be compensated for. The data for driving the actuating members is determined either by calculation or by measurement in a test run of the printing machine.
German Patent Application DE 199 14 627 A1 relates to a method and a device for compensating for a rotational oscillation in a printing machine, the method being designed in such a manner that at least one intrinsic oscillation shape of the printing machine is determined and that a specific counter-torque for compensating for the torques that excite oscillations in the intrinsic shape is applied to at least one location at which this intrinsic shape does not have the amplitude 0.
Further related art is constituted by German Patent Application 197 49 134 A1, wherein an active oscillation damping device and a method for identifying the transfer function in an active oscillation damping device are disclosed. A control device reads in a residual oscillation signal from a residual oscillation detector of the active oscillation damping device synchronously with a predetermined input sampling timer. Subsequent to reading the residual oscillation signal as a time series for each frequency, an FFT (Fast Fourier Transformation) is carried out for each time series to obtain a frequency component of the original sinusoidal wave. A calculation of the inverse FFT is then carried out for the result of the composition of each obtained frequency component in order to derive a pulse response as a transfer function.
Furthermore, European Patent EP 0 425 352 B1 describes a device for actively damping oscillations whose energies are concentrated in frequencies including a fundamental and the harmonic thereof. The device, which is used for damping oscillations of a mechanical part, includes vibration pick-ups, which pick up characteristic electrical signals, in amplitude and phase, of the vibration at a point of the mechanical part, at least one actuator which is capable of exerting a force on the mechanical part against the vibration, and an arithmetic unit which is connected to the vibration pick-up and to the actuator. The output signals of each vibration pick-up are subjected to a synchronous detection with the aid of reference signals which correspond to the different frequencies which include an energy concentration. To this end, each output signal, after being sufficiently amplified, is subjected to a sampling and to an analog-to-digital conversion for each frequency generated by a synthesizer. The random samples experience a synchronous demodulation including a multiplication and a passage through a low-pass filter and for each retained frequency. The reference signals are obtained with unaltered phase utilizing a linear relation with the fundamental frequency. For each frequency at which energy is concentrated in the mechanical oscillation, a recursive adaptation algorithm is carried out by the arithmetic unit in such a manner that each actuator receives a signal of its own which includes the sum of the contributions of the different frequencies.
German Patent Application 196 14 300 A1 discloses a method for the self-regulating compensation for the effects of the irregular concentric running of a roller or a reel for collar eccentricity compensation in a reel system or the winding up or unwinding of sheet-like material. In this context, the roller rotates at a changed speed due to the varying collar radius or roller radius. The actual draw value is approximated by at least one rotary-harmonic sine function of exclusively integral order whose argument is the rotational angle of the roller, the sinusoidal approximation being carried out according to the orthogonal correlation or in accordance with the harmonic analysis according to Fourier, and estimates being calculated for the amplitude and phase of the sinusoidal signal caused by the irregular concentric running. An additional torque which is calculated from the estimates is added to the setpoint torque value for the roller.
The determination of suitable compensation data, in particular for printing machines, constitutes a problem of compensation methods and compensation devices heretofore, which impedes the broad application thereof. If a storage device is used for the compensation data, then, in common compensation approaches heretofore, a calculation or a test run for measuring suitable compensation data are required in advance. Both methods turn out to be difficult, time and cost intensive. Typical compensation approaches in printing machines consider oscillations including a plurality of frequencies as a whole. Because of this, it is possible only with difficulty to adapt the compensation in a differentiated manner as a function of the machine dynamics, control, disturbance, or the like. In particular, an adaptation to oscillation shapes which vary strongly over time or to a changed machine dynamics can be implemented only with difficulty.
When calculating the oscillations, there is a risk of systematic errors if simplifying or even false assumptions are made. An approach of that kind implies that only oscillations which are accessible to calculation can be compensated for. Moreover, conventional test runs for determining the oscillation involve the risk of measuring errors due to different, further disturbances. When utilizing the determination of the intrinsic shapes of a machine, a machine-specific design is required.
An object of the present invention is to reduce unwanted oscillations, also referred to as disturbances, at least one shaft or virtual machine shaft of a machine rotating at a substantially constant speed.
The present invention provides a method for compensating for a mechanical oscillation (2), in particular for a rotational oscillation, which has a frequency spectrum which can be approximately represented by a number of discrete frequency components, at a machine shaft, in particular in a printing unit or in a printing machine, via at least one actuator (11) acting directly or indirectly upon the machine shaft. Upon at least one of the discrete frequency components of the mechanical oscillation (2) is superposed by the actuator (11), independently of the other frequency components, at least one substantially harmonic torque (14) of the same frequency having a specific amplitude and phase in such a manner that the amplitude of the oscillation of the machine shaft at this frequency is reduced.
The present invention also provides a printing unit having at least one machine shaft driven via an actuator (11) and a measuring device (5) for picking up the mechanical oscillation (2) of the machine shaft, wherein the printing unit features a filter (7) or a correlator (7) for the purpose of determining the amplitudes and phases of a number of discrete frequency components of the frequency spectrum of the mechanical oscillation (2).
The present invention further provides a printing unit group having at least one machine shaft driven via an actuator (11) and a measuring device (5) for picking up the mechanical oscillation (2) of the machine shaft, wherein the printing unit group features a filter (7) or a correlator (7) for the purpose of determining the amplitudes and phases of a number of discrete frequency components of the frequency spectrum of the mechanical oscillation (2).
Existing disturbances in the form of mechanical oscillations at a shaft exhibit a frequency spectrum which can approximately be represented by a number of discrete frequency components. The frequencies developing in the process are typically discrete and approximately constant but depend on the machine speed, thus having a fixed order, or are independent thereof As mentioned before, excitations corresponding to just the rotational frequency or to multiple thereof are generally not disturbing in printing machines but can also be eliminated using the method according to the present invention. The equivalent applies to virtual machine shafts, that is to a signal which is calculated via a (preferably linear) relation from the position, speed, or acceleration of one or a plurality of real shafts, the signal being in particular the difference of the coordinates of two real shafts.
The method according to the present invention for compensating for a mechanical oscillation or for a disturbance, in particular for rotational oscillations, which has a frequency spectrum which can be approximately represented by a number of discrete frequency components, at a machine shaft, in particular in a printing unit, via at least one actuator acting directly or indirectly upon the machine shaft is characterized in that discrete frequencies of this mechanical oscillation are compensated for separately. These can be both fixed frequencies and fixed orders, in other words, frequencies which bear a fixed ratio to the rotational frequency of the shaft. The actuator, which acts directly or indirectly upon the machine shaft, superposes upon each discrete frequency component of the mechanical Em oscillation, independently of the other frequency components, at least one substantially harmonic torque of identical frequency having a specific amplitude and phase in such a manner that the amplitude of the oscillation of the machine shaft is reduced for this frequency. In other words, the compensation is carried out in that, for each frequency component to be compensated for of the frequency spectrum separately, a substantially harmonic torque determined by amplitude and phase, that is a sinusoidal or cosinusoidal torque of the same frequency having a specific amplitude and phase is superposed to a driving torque in such a manner that the component of the oscillation having this discrete frequency is reduced at the machine shaft. In this context, the compensation torque can be exerted by an arbitrary actuator, in particular by a motor which acts directly or indirectly upon the machine shaft anyway.
The method according to the present invention also permits a compensation for frequencies which do not bear an integral ratio to the rotational frequency of the machine shaft (asynchronous oscillation) by treating them as synchronous oscillations referred to a frequency which is in a fixed ratio to the rotational frequency of the machine shaft. As a rule, the fixed relation is fractional rational, that is a number from the rational numbers if the disturbance is caused by a machine part which is coupled via a gearing.
The method according to the present invention, together with the printing unit according to the present invention have a number of advantages.
Since the method in each case compensates for specific discrete frequency components, measurements of the amplitude and phase of the mechanical oscillation of the machine shaft at least one specific point in time are required only for one particular frequency. Because of this, the method is virtually unaffected by other frequencies or also stochastic disturbances of the mechanical oscillation. Via the iteration of measurements related to a discrete frequency over time, the in each case instantaneous amplitudes and phases of the discrete frequency component of the mechanical oscillation are determined so that a compensation which is close in time and adapted to the respective prevailing situation is made possible.
The method according to the present invention permits a simple compensation for anharmonic periodic oscillations with an associated angular frequency xcfx89 from the relation xcfx89=2xcfx80/T. where T denotes the oscillation period, in that, in each case separately, the harmonic oscillations of angular frequencies nxc3x97xcfx89, with n being a natural number, which have specific amplitudes and phases and represent the anharmonic oscillation, are compensated for. Likewise, the method according to the present invention makes it possible to take into account the amplifications and phase shifts of the different frequency components used for compensation from the actuator to the compensation shaft. Known compensation methods heretofore which work with non-harmonic compensation torques, as are usually stored in storage devices, can take into account this dependence of the transmission on the machine dynamics only with difficulty since the curve shape stored in the storage device defines the amplitudes and phases of the harmonic waves with respect to the fundamental wave. However, the determination of the amplitudes and phases of individual frequency components is flexible. Thus, the method according to the present invention eliminates diverse limitations of existing compensation methods and, in this manner, makes a compensation easier in practical use.
Moreover, the method according to the present inventions makes do without calculating the mechanical oscillation of the machine shaft. Because of this, the risk of systematic errors in the case of simplifying or false assumptions is averted; an extensive modeling of the machine or a computational effort by specialists are dropped.
The method according to the present invention has the further advantage of being usable both for unsteady oscillations, that is oscillations which vary over time, for a machine state which varies over time, for synchronous and asynchronous oscillations, or also for oscillations having a constant frequency. Individual frequency components which are in a complex relation to the machine frequency, whether they are sidebands of oscillations or oscillations resulting from modulation, can also be compensated for using the method according to the present invention.
In principle, it is possible for an arbitrary or a necessary number of different frequency components to be compensated for using the method according to the present invention since the compensations do not mutually influence one another. For instance, only the dominating, largest disturbing frequency components or orders have to be known or to be measured in amplitude and phase to permit the compensation therefor. Such a concentration on the compensation of the most disturbing frequencies permits considerable improvements already with little effort. Since different orders or frequency components can be compensated for as a function of the machine speed, it is advantageously possible, for example, to always compensate for all oscillation frequency components near the resonant frequency of the machine or of parts of the machine.
Knowing the transfer function, it is also possible to feed in at least one compensation torque at a large distance from the machine shaft to be balanced. Generally, the method is rugged with respect to changes in the machine dynamics such as the transition from the uncontrolled to the controlled machine or the selection of different control parameters or controls.
Using the method or the printing unit according to the present invention, it is possible for the quality of the products to be considerably increased since an improved, from rotation to rotation identical curve of the speed or position as a function of the angle of the machine shaft is possible. The method according to the present invention allows the separation points of mechanically decoupled sheet-fed printing presses to be positioned with considerably greater repeat accuracy than with conventional methods.